Conventional gain-controlled amplifiers of the above mentioned general type have at least one of the following drawbacks, i.e.
they require high supply power and are thus not suitable for battery powered units; PA1 they are not suitable for high operating frequencies; PA1 the signal handling capability in particular in respect to linearity is degraded if the gain is reduced; and/or such amplifiers PA1 exhibit poor noise figures.
The objects of the invention are to eliminate such problems and to provide a gain-controlled RF signal amplifier suitable for high operating frequencies in the GHz range with good operating linearity, in particular if the gain is reduced, having low noise figure and low power consumption.
These objects are solved according to the invention by a gain-controlled RF signal amplifier provided with an adaptively controlled feedback network according to the characterizing part of claim 1.
Advantageous amendments, improvements and/or embodiments of the subject matter of claim 1 are defined by dependent claims, are described in the following part of this specification and/or are within the scope of the skill of the average expert after having educated himself/herself by reading the present disclosure of the invention.
Variable gain RF amplifiers with more or less linear gain control are known in the art, e.g. in a certain configuration using a dual-gate FET wherein the second gate is used for gain control.
EP-A2-0 601 888 discloses a gain-controlled RF amplifier using a PIN-diode in a series feedback circuit. A similar structure for a gain-controlled RF amplifier using a PIN-diode as a load altering resistance is the subject matter of U.S. Pat. No. 4,275,362. U.S. Pat. No. 4,057,765 describes a variable gain RF input amplifier in which the first 10 dB of a signal attenuation are effected by an AGC voltage applied to the amplification transistor followed by a supplementary attenuation by an increasing reverse bias applied to a PIN-diode in the signal input path to the amplification transistor. The PIN-diode in this circuit is only used as a direct regulation means for higher attenuation values.
Other examples of gain-controlled RF signal amplifiers, more remote from the invention, are the subject matters of U.S. Pat. No. 4,147,991 and U.S. Pat. No. 4,019,160.